This invention relates generally to paper making machines and, more particularly, to a method and apparatus for achieving reliable transport of a web from the press section to the initial stages of the drying section of a paper making machine.
Recently, paper making machines have been designed to operate at greater velocities and to produce webs of greater width then previous similar machines. Problems have become apparent in the operation of such new paper making machines, particularly at the initial stages of the drying section and, in particular, it has not been uncommon for a web to rupture during such operation, especially during the start up period of the machine. Such web ruptures have resulted in serious decreases in production efficiency. In order to overcome these problems, multiple cylinder drying sections have been under development which support the web with a drying wire or felt when the web is carried in the first cylinder group of the drying section. Typical examples of such multiple cylinder drying sections are disclosed in U.S. Pat. Nos. 3,874,997; 2,091,805; 3,503,139; and 3,576,078.
In U.S. Pat. No. 3,874,997, a multiple cylinder dryer section is disclosed wherein the web is carried in closed relationship and supported by a dryer fabric and in which both contact and flow through drying is accomplished by providing the drying cylinders of one cylinder row to be pervious to a dry gas stream and by locating special hoods close to these drying cylinders from which the drying gas stream is conducted through the web, the supporting wire and the perforated drying cylinder mantel.
U.S. Pat. No. 2,091,805 discloses a multiple drying cylinder section arranged to provide a closed or supported transport of the web in order to avoid free or unsupported draw. In the construction of the apparatus disclosed in this patent where the web travels over a zig-zag path from one drying cylinder row to another while supported by a wire, another row of suction transport cylinders equipped with pervious mantels is provided. In such embodiments, it is necessary to provide drying cylinders with pervious mantels and vacuum or overpressure chambers in at least one cylinder row. However, such drying cylinders equipped with pervious mantels and vacuum and overpressure equipment are relatively expensive.
U.S. Pat. No. 3,503,139 discloses a dryer section for a paper making machine including a multiple cylinder dryer which does not require the use of flow through cylinders of the type described hereinabove. In the structure disclosed in this patent, the web is transported from one row of the cylinder group to another while continuously supported by the wire. However, in such construction, the temperature of the upper cylinder row is usually higher than normal since in the use of such apparatus a wire or felt is located between the web and the surface of the drying cylinder. Thus, at high speeds, the paper web may not be reliably held on the outer surface of the felt.
Similarly, in U.S. Pat. No. 3,576,078, a multiple cylinder dryer section is disclosed having a pair of wires between which the web is transported over the entire drying cylinder section. Although in the apparatus disclosed therein, flowthrough cylinders are not required, the use of such apparatus has not been entirely satisfactory since the web must on both the upper and lower drying cylinders continuously run between two wires. This results in the disadvantage at the portion of web travel wherein the web is bent or the direction of travel changes, the velocity of the web enclosing wires differ causing rubbing and abrasion thereby impairing the quality of the web surface.
It has also been proposed to move a narrow belt located in the side area adjacent the drying cylinders onto the top of the web utilizing special equipment while the web passes over the upper row of drying cylinders.